The present invention relates to electronically tunable optic filters utilizing non-collinear interaction of light and sound beams in an optically anisotropic medium. The term "non-collinearity" defined in this disclosure refers to the energy flow direction (i.e., group velocity) of the optical and acoustic beams.
Electronically tunable acousto-optic filters have been constructed utilizing a polarized light beam propagating collinearly with an acoustic beam in an anisotropic medium. The incident light beam of a first polarization is diffracted by the acoustic wave into a second polarization that may be separated from the incident beam with crossed polarizers. At a fixed acoustic frequency, the diffraction occurs only for a band of optical frequencies around a center optical wavelength determined by the acoustic frequency. When the acoustic frequency is changed, the center wavelength of the optical passband of the filter is correspondingly changed. Such a collinear acousto-optic filter is disclosed in a patent application entitled "Tunable Acousto-Optic Method and Apparatus" filed by Stephen E. Harris and issued as U.S. Pat. No. 3,679,288 on July 25, 1972. Harris was concerned primarily with acoustic wave propagations that are isotropic, i.e., the acoustic wave phase velocity and group velocity are coincident. Thus in the collinear acousto-optic filter described by Harris, both the acoustic beam (i.e., group velocity of the acoustic wave) and the acoustic wavefront normal (phase velocity) are collinear with the optical beam.
The collinearity requirement of Harris' filter brought the two most important benefits: (1) The interaction volume of the acoustic and optical beams is enhanced, thus the drive power for maximum filter transmission is drastically decreased. (2) The angular aperture of the filter is large compared with conventional isotropic Bragg cell.
The above-mentioned patent also gave a mathematical description of the collinear acousto-optic interaction in an anisotropic medium and described the characteristics of the collinear acousto-optic filter that included filter tuning relation (i.e., center wavelength of the filter passband vs. acoustic frequency), transmission and drive power, filter spectral bandwidth and angular aperture. It also described means of coupling of optical and acoustic beams into the filter medium and separation of the filtered light from the incident light beam.
The acoustic wave propagations in some crystals are anisotropic; i.e., the acoustic phase velocity and group or energy velocity are noncollinear. The acoustic beam "walks-off" from the acoustic wavefront normal. For instance, the angle between the phase and group velocities for a shear wave propagating along the y-axis in crystal quartz is equal to 24.3.degree.. For such cases the optical beam cannot be simultaneously collinear with both the phase and group velocities of the acoustic beam. In a patent application entitled "Electronically Tunable Acousto-Optic Filter Having Improved Light and Acoustic Wave Interaction" filed by John A. Kusters and issued as U.S. Pat. No. 3,687,521 on Aug. 29, 1972, a filter construction is described in which the group velocity of the acoustic wave is chosen to be collinear with the optical beam. This configuration has the advantage of maximizing interaction length and consequently the low drive power. Also described in the patent are means of coupling the optical and acoustic beams so that the acoustic group velocity and the optical beam can be collinear inside the interaction medium.
The configuration described in the above-mentioned patent has an undersirable result. The angular aperture of the filter becomes very small. Or equivalently, the passband of the filter is broadened for larger angular apertures. As described in a patent application filed by Donald L. Hammond, John A. Kusters and David A. Wilson and issued as U.S. Pat. No. 3,756,689 on Sept. 4, 1973, certain crystal orientations exist where the effect of birefringence change is cancelled out by the effect due to acoustic anisotropy. Choosing these directions for acousto-optic filters in the configuration described in the above-cited U.S. Pat. No. 3,687,521, enhanced resolution and improved angular aperture is obtained. Also described in the U.S. Pat. No. 3,756,689 are the specific crystal orientations for crystal quartz and LiNbO.sub.3.
In all of the above-cited patents (U.S. Pat. Nos. 3,679,288; 3,687,521 and 3,756,689) the configuration of the acousto-optic filters are described so that the acoustic beam (i.e., the group or energy velocity of the acoustic wave) is substantially collinear with the light.